1. Field of the Invention
This invention relates to polyamide compositions, and to a process of preparing same.
2. Description of the Related Art
Thermoplastic resins used as plastics in engineering applications can require additives to modify, that is, increase the impact resistance of said thermoplastic resins. U.S. Pat. No. 3,845,163 discloses blends of 60 to 85 weight percent polyamide and an acid-containing olefin polymer. U.S. Pat. No. 3,388,186 discloses a polyamide composition that includes an ethylene-containing grafted copolymer. U.S. Pat. No. 3,465,059 similarly discloses a polyamide composition that includes an ethylene-containing grafted copolymer. U.S. Pat. No. 3,668,274 discloses polycarbonamides that have been modified with an elastomeric phase and a rigid phase thermoplastic stage containing amine-reactive moieties.
Maleic anhydride grafted polyolefins (maleated polyolefins) and ethylene copolymers are widely used as impact modifiers for engineering polymers, particularly for polyamide polymer resins. Maleated polyolefins can be the preferred modifiers for use with polyamide resins. U.S. Pat. No. 4,174,358 discloses copolymers of ethylene/maleic anhydride as tougheners for polyamides having a number average molecular weight of at least 5000. U.S. Pat. No. 5,346,963 describes the use of maleic anhydride-grafted substantially linear ethylene polymers and blends thereof with olefin polymers and also with non-olefin polymers such as polyamides. U.S. Pat. No. 6,008,297 describes polyamide compositions that include acid-grafted ethylene/alpha-olefin copolymers for improved low-temperature impact strength.
The use of maleated polyolefins is not trouble-free, however. There are limitations with using maleated polyolefins as impact modifiers. For example, it can be difficult to reach a high level of grafting in a maleated polyolefin. Grafting, as conventionally practiced, involves the use of a free-radical initiator. To achieve higher levels of grafting, which can be desirable, it can be necessary to include the free-radical initiator in a higher concentration. Using the initiator at higher concentrations can cause undesirable side reactions, such as crosslinking and chain scission. For this reason, grafting of maleic anhydride at a concentration of greater than 2 wt % can be difficult and troublesome. In addition, controlling the quality and consistency of maleated polyolefins—particularly those having higher concentrations of maleic anhydride grafting—can be troublesome. By-products, unreacted monomers, and crosslinking can significantly detract from the quality of maleated polyolefins at any grafting level, but this can be more of a problem particularly as the grafting level increases. Further, the cost of producing maleated polyolefins with high maleic graft levels can be substantial.
Ethylene copolymers that include reactive functional groups such as maleic anhydride can be readily obtained by a high-pressure free radical process. However, these copolymers generally are not good impact modifiers for polyamide polymer resins.
It would be desirable to prepare an impact modifier for polyamide polymer resins that are effective tougheners for polyamide resins and do not have the problems associated with high levels of grafting in maleated polyolefins.